As a result of the steadily increasing integration density of semiconductor devices, photolithographic masks or templates for nanoimprint lithography have to transfer smaller and smaller features. To meet this demand, the actinic wavelength of the imaging tool has been reduced in steps of 436 nm and 365 nm to 193 nm. Furthermore, immersion lithography has been introduced to enlarge the numerical aperture of the projection systems. As a consequence, the mask manufacturing process has reached a very high degree of complexity accompanied with strongly enlarged costs. In the near future, reflective masks will be used for imaging using light of a wavelength of 13.5 nm.
In order to manufacture photolithographic masks with a sufficient yield, mask defects are removed by repairing at the end of the manufacturing process. The generation of structure elements on the wafer illuminated with the photolithographic mask may also lead to small defects which have also to be corrected. Moreover, ultralarge-scale (ULSI) chips often have a multitude of integrated circuit variants on the same chip which are selected, activated and repaired by circuit editing. In addition, the advances of the microlithography technique allow the fabrication of for example micro-electro-mechanical systems (MEMS) or photonic integrated circuits (PIC) having smaller and smaller feature sizes whose fabrication is an error-prone process. Similar to the situation of the photolithographic masks, it is necessary to correct the errors of these devices whenever possible.
In the following these items and further ones are summarized with the term substrate.
Typically, the above mentioned errors are local defects which can be corrected by using a focussed particle beam. The focussed particle beam provides the spatial resolution (<1 μm) required for the above mentioned small structures. In order to repair local defects on a substrate a suitable processing gas is applied in combination with the particle beam in order to induce particle beam assisted chemical processes. For locally removing excessive material of a substrate, the processing gas comprises at least one etching gas. In case of locally depositing a certain material, a precursor gas or a chemical vapour deposition (CVD) gas is used as processing gas.
During the processing of a defect, the defect may heat up. Further, the micromanipulators used for scanning may acquire some slip within a certain time period. Furthermore, when using a charged particle beam, the substrate surface may charge up, and thus leading to shifts and/or distortions of the position where the charged particle beam hits the substrate. All these effects lead to a drift of the relative position between the incident particle beam and the substrate to be processed which deteriorates the spatial resolution of the particle beam with respect to the defective area of the substrate.
This problem can be solved by using and/or fabricating and using a reference mark close to the defect on the substrate. In some embodiments, the reference mark has dimensions in the range of 50 nm to 100 nm. During the repairing process, the reference mark is used to correct the drift of the particle beam with respect of the defective area. In the prior art, such a process is called drift correction (DC).
The following documents should be noted as prior art for the present patent application: U.S. Pat. No. 7,018,683, EP 1 662 538 A2, JP 2003007247 A, US 2007/0023689, and US 2008/0073580.
However, a new situation occurs if the size of feature elements is reduced to such an extent that their dimensions become comparable with the dimensions of the reference mark(s). Then, the reference mark(s) can no longer be ignored as they can influence the further processing of the substrate and there is a danger that the reference mark(s) can have an effect on an image of the substrate.
It is therefore one object of the present invention to provide a method and an apparatus for minimizing the effect of a reference mark on the further processing and/or on the application of the substrate.